The present invention relates to a shaft seal for a pump or an agitator, and to a pump or an agitator, which for convenience are generally referred to hereinafter as a pump, comprising the shaft seal. In particular, the invention relates to a shaft seal for a pump of the type comprising a pump housing defining a fluid pumping chamber, an impeller located in the pumping chamber, a pump shaft carrying the impeller for rotating the impeller in the pumping chamber for pumping fluid through the pumping chamber, and a shaft accommodating bore extending through the pump housing into the pumping chamber for accommodating the pump shaft into the pumping chamber, the pump shaft being axially movable in the shaft bore for adjusting an axial clearance gap between a face of the impeller and an adjacent face of the pump housing in the pumping chamber. The respective adjacent faces of the impeller and the pump housing which define the clearance gap may extend radially relative to the rotational axis of the pump shaft or at an angle other than 90xc2x0 to the rotational axis to define a frusto-cone.
Such pumps are well known, and in general, they are used for pumping slurry type fluids, typically of the type in which abrasive particular matter is entrained. Typically, they are of a type having an axial inlet and a radial outlet. The axial clearance gap between the impeller and the housing in the pumping chamber is typically provided at the axial end of the impeller adjacent the inlet. It is important that the axial clearance gap be maintained as small as possible in order to provide the maximum pressure drop across the impeller, while at the same time providing sufficient clearance for the impeller to rotate within the pumping chamber. Depending on the size of pump and the desired pressure drop across the impeller, and indeed, the fluid being pumped, the axial clearance gap between the impeller and the pump housing may vary up to approximately 1 mm.
However, the face of the impeller which defines the clearance gap in such pumps is prone to wear depending on the abrasive nature of the slurry fluid, and the rate of wear is a function of the degree of abrasiveness of the slurry material. As the face of the impeller wears, the axial clearance gap between the impeller and the pump housing opens up, thereby leading to an undesirable reduction in the pressure drop across the impeller. In order to avoid the need for replacing the impeller each time the axial clearance gap has opened up to an undesirable gap width, adjustment is provided in such pumps which permits the pump shaft to be axially movable within the shaft accommodating bore. Adjustment is undertaken by urging the pump shaft axially within the shaft bore until the face of the impeller abuts the adjacent face of the pump housing, thereby closing the axial clearance gap between the impeller and the pump housing. The axial clearance gap is then adjusted to the desired gap width by urging the pump shaft in the opposite axial direction an appropriate amount, thereby urging the impeller from the pump housing to open up the axial clearance gap to the desired gap width, typically up to 1 mm.
A shaft seal is provided around the pump shaft in the shaft bore for sealing the pumping chamber from the shaft bore. Such shaft seals, in general, comprise a combination of a stationary annular sealing bushing which is located in the shaft bore adjacent the pumping chamber and extends around the pump shaft, and a packed gland seal which is located spaced apart from the stationary sealing bushing downstream thereof relative to the pumping chamber. A spacing means typically a lantern ring is located in the fluid cavity for maintaining the packed gland seal spaced apart from the stationary sealing bushing. The stationary sealing bushing defines a circumferentially extending radially facing sealing surface which co-operates in sealing relationship with a corresponding radially facing circumferential sealing surface of the pump shaft. The packed gland seal defines with the pump shaft, the pump housing and the stationary sealing bushing an annular cavity which accommodates a pressurised lubricating fluid for lubricating the adjacent sealing surfaces of the stationary sealing bushing and the pump shaft. A charging port is provided into the fluid cavity for pumping the lubricating fluid under pressure into the cavity so that the pressure of the lubricating fluid in the fluid cavity is at a positive pressure relative to the slurry being pumped through the pumping chamber. This, thus, to a large extent prevents flow of slurry past the stationary sealing bushing, and permits the lubricating fluid to lubricate the adjacent sealing surfaces of the stationary sealing bushing and the pump shaft. The packed gland seal seals, the fluid cavity and prevents flow of the lubricating fluid between the pump shaft and the pump housing in a downstream direction relative to the pumping chamber.
Unfortunately, such arrangement of seals in such pumps are unsatisfactory. Firstly, although the lubricating fluid, which is typically water, is maintained at a positive pressure relative to the slurry being pumped in the pumping chamber, slurry does leak past the stationary sealing bushing, thereby causing wear both to the stationary sealing bushing and the shaft. In order to avoid having to replace the pump shaft at regular intervals, a wear sleeve is generally provide around the pump shaft adjacent the annular sealing bushing, which can be replaced when the annular sealing bushing is being replaced when wear becomes excessive. However, in order to replace the annular sealing bushing, it is necessary to disassemble the pump. The pump shaft must be withdrawn, and the entire seal around the pump shaft disassembled. The impeller must also be disassembled from the pump shaft to permit replacement of the wear sleeve. After the wear sleeve has been replaced on the pump shaft and the pump shaft and impeller assembled in the pump, the shaft seal again has to be assembled by inserting a new stationary sealing bushing, the lantern ring, and the packed gland seal. This is a time consuming task and also requires the services of skilled fitters, and thus, as well as being time consuming, is also expensive. An alternative sealing arrangement for sealing the pump shaft of such pumps includes the provision of a packed gland seal only. However, in such cases a shoulder must be provided within the shaft bore for acting as an inner abutment surface for the packed gland seal, and the radial clearance gap between the shoulder and the pump shaft must be relatively small to avoid the packing material of the packed gland extending beyond on the shoulder. In general, the radial gap opens up as a result of wear between the shoulder and the pump shaft caused by the slurry. Once the radial gap has opened sufficiently, the gland packing material commences to extrude between the shoulder and the pump shaft into the pumping chamber. This, thus, reduces the sealing pressure in the packed gland, thereby leading to slurry leakage past the packed gland seal. Further tightening of the packed gland seal merely exacerbates the problem, since it extrudes further material from the packed gland seal between the shoulder and pump shaft, and also increases frictional resistance on the pump shaft. This is undesirable. A similar problem can arise with a shaft seal of the type described above which comprises a stationary sealing bushing and a packed gland seal spaced apart from the stationary sealing bush when the slurry being pumped leaks into the lubricating fluid cavity. Such leakage of slurry into the lubricating fluid cavity causes wear between the pump shaft and the lantern ring, thereby permitting the packing material of the packed gland seal to extrude between the lantern ring and the shaft.
There is therefore a need for a shaft seal for such a pump, and indeed, for any other type of pump which overcomes these problems, and there is also a need for a pump comprising such a shaft seal.
The present invention is directed towards providing such a shaft seal and a pump.
According to the invention, there is provided a shaft seal for a pump of the type comprising a pump housing defining a fluid pumping chamber, an impeller located in the pumping chamber, a pump shaft carrying the impeller for rotating the impeller in the pumping chamber for pumping fluid through the pumping chamber, and a shaft accommodating bore extending through the pump housing into the pumping chamber for accommodating the pump shaft into the pumping chamber, the pump shaft being axially movable in the shaft bore for adjusting an axial clearance gap between a face of the impeller and an adjacent face of the pump housing in the pumping chamber, the shaft seal comprising a stationary sealing ring for mounting in the shaft bore and having a stationary sealing surface, a rotating sealing ring for mounting on the pump shaft and having a rotating sealing surface for co-operating with the stationary sealing surface for forming a seal from the pumping chamber between the pump shaft and the pump housing in the shaft bore, wherein the stationary and rotating sealing surfaces face in respective generally axial directions opposite to each other, and one of the stationary sealing ring and the rotating sealing ring is axially movable relative to the corresponding one of the pump housing and the pump shaft, and a means is provided for axially moving the movable one of the stationary sealing ring and the rotating sealing ring with the other of the stationary and the rotating sealing rings during axial movement of the pump shaft and the pump housing relative to each other for adjusting the axial clearance gap in the pumping chamber between the impeller and the pump housing so that on the relative axial movement of the pump shaft and the pump housing for opening the axial clearance gap to the desired gap width, the respective stationary and rotating sealing surfaces are brought into sealing relationship with each other.
In one embodiment of the invention, the means for moving the movable one of the stationary sealing ring and the rotating sealing ring is provided for moving the said movable sealing ring with the other sealing ring as the relative axial movement of the pump shaft and the pump housing is in a direction for closing the axial clearance gap between the impeller and the pump housing.
In another embodiment of the invention, the means for moving the movable one of the stationary sealing ring and the rotating sealing ring is provided for moving the movable sealing ring a sufficient distance with the other sealing ring so that on relative axial movement of the pump shaft and the pump housing to open the axial clearance gap between the impeller and the pump housing to the desired gap width, the respective stationary and rotating sealing surfaces are brought into the sealing relationship with each other.
Preferably, the means for moving the movable one of the stationary and rotating sealing rings comprises respective first and second abutment means for co-operating with each other for moving the movable one of the stationary and rotating seal rings with the other sealing ring on relative axial movement of the pump shaft and the pump housing for closing the axial clearance gap between the impeller and the pump housing, the first abutment means being provided on the movable one of the stationary and rotating sealing rings, and the second abutment means being located on the one of the pump housing and the pump shaft on which the other of the sealing rings is mounted.
In one embodiment of the invention, the stationary sealing ring is axially movable in the shaft bore.
Preferably, the first abutment means defines an axially facing first abutment surface on the stationary sealing ring, and the second abutment means defines a second axially facing abutment surface on the pump shaft facing in the general direction of axial movement of the pump shaft for closing the axial clearance gap between the impeller and the housing for abutting the first abutment surface when the pump shaft is moved in the axial direction for closing the clearance gap between the impeller and the pump housing.
Advantageously, the first abutment surface extends circumferentially around the stationary sealing ring, and preferably, the first abutment surface is defined by an annular flange extending inwardly from the stationary sealing ring.
Advantageously, the second abutment surface extends circumferentially around the pump shaft, and preferably, the second abutment surface is defined by a step change in the diameter of the pump shaft.
In one embodiment of the invention, the second abutment surface is defined by a sleeve rigidly mounted on the pump shaft.
In one embodiment of the invention, the spacing between the respective first and second abutment surfaces when the stationary and rotating sealing surfaces are in sealing relationship should not exceed the desired gap width of the axial clearance gap between the impeller and the pump housing.
In another embodiment of the invention, the stationary sealing ring comprises a stationary seal carrier ring for carrying an annular stationary seal. Preferably, the annular stationary seal is mounted in the stationary seal carrier ring.
In another embodiment of the invention, the rotating sealing ring comprises a rotating seal carrier ring for carrying an annular rotating seal. Preferably, the annular rotating seal is mounted in the rotating seal carrier ring.
In a further embodiment of the invention, a packed gland seal is provided for locating in the shaft bore spaced apart from the stationary and rotating sealing rings, the packed gland seal defining with the pump housing, the pump shaft and the stationary and rotating sealing rings an annular fluid cavity for a lubricating fluid for lubricating the stationary and rotating sealing surfaces of the respective stationary and rotating sealing rings, the annular fluid cavity being located on the downstream side of the stationary and rotating sealing rings relative to the pumping chamber.
In one embodiment of the invention, an access means is provided for pressurising the lubricating fluid in the fluid cavity so that the lubricating fluid has a positive pressure relative to the fluid in the pumping chamber.
Preferably, a spacing means is provided for spacing the packed gland seal apart from the stationary and rotating sealing rings for forming the fluid cavity.
Additionally, the invention provides a pump comprising a pump housing defining a fluid pumping chamber, an impeller located in the pumping chamber, a pump shaft carrying the impeller for rotating the impeller in the pumping chamber for pumping fluid through the pumping chamber, a shaft accommodating bore extending through the pump housing into the pumping chamber for accommodating the pump shaft into the pumping chamber, the pump shaft being axially movable in the shaft bore for adjusting an axial clearance gap between a face of the impeller and an adjacent face of the pump housing in the pumping chamber, and a shaft seal located in the shaft bore around the pump shaft for sealing the shaft bore from the pumping chamber, wherein the shaft seal is a shaft seal according to the invention.
Furthermore, the invention provides a pump comprising a pump housing defining a fluid pumping chamber, an impeller located in the pumping chamber, a pump shaft carrying the impeller for rotating the impeller in the pumping chamber for pumping fluid through the pumping chamber, a shaft accommodating bore extending through the pump housing into the pumping chamber for accommodating the pump shaft into the pumping chamber, the pump shaft being axially movable in the shaft bore for adjusting an axial clearance gap between a face of the impeller and an adjacent face of the pump housing in the pumping chamber, a stationary sealing ring mounted in the shaft bore and having a stationary sealing surface, a rotating sealing ring mounted on the pump shaft and having a rotating sealing surface for co-operating with the stationary sealing surface for forming a seal between the shaft bore and the pumping chamber, wherein the stationary and rotating sealing surfaces face in respective generally axial directions opposite to each other, and one of the stationary sealing ring and the rotating sealing ring is axially movable relative to the corresponding one of the pump housing and the pump shaft, and a means is provided for axially moving the movable one of the stationary sealing ring and the rotating sealing ring with the other of the stationary and the rotating sealing rings during axial movement of pump shaft and the pump housing relative to each other for adjusting the axial clearance gap in the pumping chamber between the impeller and the pump housing so that on the relative axial movement of the pump shaft and the pump housing for opening the axial clearance gap to the desired gap width, with the respective stationary and rotating sealing surfaces are brought into sealing relationship with each other.
In one embodiment of the invention, the pump is suitable for pumping abrasive type fluids.
In another embodiment of the invention, the pump is suitable for pumping slurry type fluids.